1 / 14

140 likes | 296 Views

Psychrometrics – Lecture 1. Reading: Chapter 9 in Henderson/Perry Handouts. Dr. C. L. Jones Biosystems and Ag. Engineering . Psychrometry. Study of physical and thermal properties of air and water vapor mixtures. Simply put: the science of drying. Dr. C. L. Jones

Download Presentation
## Psychrometrics – Lecture 1

**An Image/Link below is provided (as is) to download presentation**
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.
Content is provided to you AS IS for your information and personal use only.
Download presentation by click this link.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

**Psychrometrics – Lecture 1**• Reading: • Chapter 9 in Henderson/Perry • Handouts Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometry**• Study of physical and thermal properties of air and water vapor mixtures. • Simply put: the science of drying Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – Ideal Gas Law**• Where: p = absolute pressure V = volume N = number of kg-moles R = universal gas constant T = absolute temperature (K) Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – Ideal Gas Law**• For a fixed number of moles of an ideal gas: • p1V1/T1=p2V2/T2 • For Mixtures…Dalton’s Law pg. 238 Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – Definition of Terms**• Atmospheric air is always a mixture of dry air and water vapor. Physical and thermal quantities describe the state of the mixture… • Dry-bulb temperature: true temp. of the mixture • Saturation pressure: pressure where liquid and vapor forms of water are in equilibrium… evaporation rate = condensation rate • Use standard steam table or calculate (9.18) ln psat = 31.9602 – 6270.3605/T – 0.46057lnT (for 255.38 to 273.16 K) ln(psat/R’) = (A + BT + CT2 + DT3 + ET4)/(FT – GT2) (for 273.16 to 2533.16 K) K = C + 274.15 Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – more terms**• Humidity ratio: mass of water vapor per unit mass of dry air • Relative humidity: measure of amount of water air can hold at a certain temperature • Specific Volume: vol. per unit mass of dry air at the state of the mixture…iow…space occupied by air…greater volume at warmer temperatures Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – yet more terms**• Dew-Point Temperature: temperature at which water will begin to condense out of moist air. (constant humidity ratio and pressure) • Wet-bulb Temperature: determined when air is circulated past a wet sensor tip. Temp. at which water evaporates and brings the air to saturation. (constant enthalpy) (adiabatic process) • Enthalpy: heat energy content of moist air Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics**• Mollier vs. Traditional charts Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics**• Mollier vs. Traditional charts pg 252-253 Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics – State Points**• Find a point using two properties… • Can find all of the other 5 properties • This point is called the “state point” • Example: 25°C dry bulb and 18° C wet bulb • Find enthalpy, sp. vol., humidity ratio, rh, dewpoint Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometrics**• Mollier vs. Traditional charts Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometric Processes Application: condensation**A carton of eggs was removed from the refrigerator and left on the kitchen counter. The refrigerator was set at a temperature of 5oC and the kitchen at 24oC, 30% relative humidity. Will condensation occur (i.e., "sweat" appear on eggs)? (def. of dew point: temperature at which water will begin to condense out of moist air) SOLUTION The corresponding dew-point temperature at 24oC and 30% relative humidity is 4.5oC. Since 4.5oC < 5oC, condensation will not occur. Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometric Processes Application: power requirement for**heaters • Example 9.10 pg. 258 • Mass flow rate m = Q/v, • Q=vol. flow rate, • v=spec. vol. of air q = m(Δh) where q = rate of heat transfer So: q = (Q/v) Δh Dr. C. L. Jones Biosystems and Ag. Engineering**Psychrometric Processes**• Application example: power requirement for heaters • Example 9.10 pg. 258 • v = 0.8724 m^3/kg, • h1 = 55.8 kJ/kg, • h2 = 76.3 kJ/kg Dr. C. L. Jones Biosystems and Ag. Engineering

More Related